As a communication standard employed for a trunk transmission path such as a backbone line, and other transmission paths, Optical Transport Network (OTN) recommended by International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is proposed. OTN defines an Optical channel Data Unit (ODU) as one of data units (or data frames), which is a unit for transmitting/receiving data.
ITU-T permits multiplexing of ODUs. For example, it is permitted to use a high-order ODU whitch includes low-order ODUs of a plurality of channels in a payload part (such as an Optical channel Payload Unit (OPU) payload part). More specifically, it is permitted to use an ODU2 which includes ODU0s of a plurality of channels in a payload part. It is also permitted to multiplex high-order ODUs of a plurality of channels, into each of which low-order ODUs are multiplexed, into a higher-order ODU. More specifically, it is permitted to further multiplex, for example, ODU2s of a plurality of channels, into each of which ODU0s of a plurality of channels are multiplexed, into an ODU4 (namely, the ODU2s are included in a payload part of the ODU4).
According to the currently proposed OTN, a communication apparatus such as a transmission apparatus, a reception apparatus or the like can not recognize a fault (in other words, an error) that occurs in low-order ODUs unless it performs synchronization at a level of the low-order ODUs when the low-order ODUs are multiplexed into a high-order ODU. Namely, the communication apparatus can only recognize a fault that occurs in low-order ODUs if it demultiplexes the low-order ODUs of a plurality of channels multiplexed into a high-order ODU and synchronizes the demultiplexed low-order ODUs. Accordingly, when ODUs are multiplexed, a plurality of synchronization circuits need to be provided within a communication apparatus according to the number of multiplexed ODUs . This causes a technical problem such that a circuit scale or a processing load of a communication apparatus increases.
The same can be said also in a case where an Ethernet (registered trademark) frame in 10 Giga bit Ethernet (GbE) or GbE, a Synchronous Transport Module (STM) frame in Synchronous Digital Hierarchy (SDH), a Synchronous Transport Signal (STS) frame or an Optical Carrier (OC) signal in Synchronous Optical Network (SONET) is mapped in a payload unit of an ODU. Specifically, when such a mapping process is executed, a communication apparatus can not recognize a fault that occurs in a frame unless it demaps a frame mapped in the ODU and synchronizes the demapped frame. Accordingly, it is needed to execute the demapping process and the synchronization process only for recognition of a fault that occurs in a frame under certain circumstances. This causes a technical problem such that a circuit scale or a processing load of a communication apparatus increases.